In the image forming apparatus, a photoconductive drum may be changed with time or relative positional deviation between respective components may occur due to an increase of temperature in an apparatus. In the image forming apparatus, when such situations occur, respective images are deviated from a reference position, a reference magnification, and a reference concentration, positional deviation or concentration deviation may be caused between respective color images overlapped with each other.
For that reason, the image forming apparatus performs image quality stabilizing processing which corresponds to position adjustment processing and concentration adjustment processing of respective color images at the time of warming up, or the like. The image forming apparatus forms, for example, test pattern of respective colors on a transfer belt, detects the test patterns by an image quality maintaining sensor, and corrects position and concentrations of respective color images based on the measured result.
In the image forming apparatus, image formation using a decolorable toner and a non-decolorable toner can be selectively conducted (for example, see JP-A-2014-174317).